1. Field of the Invention
In general, the present invention relates to systems and methods that are used to regenerate spent acids. More particularly, the present invention relates to systems and methods that are used to regenerate acids used in the pickling of metal.
2. Description of Prior Art
In the manufacturing of metal products, the process of pickling is the immersion of the metal into an acid bath. The acid bath removes the scale, oxides, and other impurities from the metal surface.
When ferrous metals, such as steel, are pickled, the pickling acid is typically hydrochloric acid. As the ferrous metal is pickled, the hydrochloric acid reacts with the ferrous metal and becomes spent. In a typical pickling reaction, where steel is pickled in hydrochloric acid, the hydrochloric acid reacts with iron oxides on the steel to produce ferrous chloride. As ferrous chloride is produced, the hydrochloric acid is consumed. Thus, as pickling continues, the pickling acid bath contains less and less hydrochloric acid and more and more ferrous chloride. Eventually, the pickling acid bath becomes so diluted that it becomes ineffective. In the pickling of steel, it is commonly accepted that iron salts, such as ferrous chloride, should be in solution at a concentration less than 12 gms/100 ml for the acid bath to be considered effective.
Once an acid bath become ineffective, the acid bath must be replaced. Spent acid baths are known in the industry as “spent pickle liquor”. Traditionally spent pickle liquor has been disposed of by being sent to a water treatment facility for neutralization or by being injected and forgotten in deep wells. An alternative to disposal, however, is regeneration. In a regeneration process, a chemical reaction is performed on the ferrous chloride in the spent pickle liquor to regenerate hydrochloric acid. The hydrochloric acid is then separated from the by-products so that it can be reused.
In the steel industry today, the dominant form or acid regeneration is roasting. During roasting, spent pickle liquor is boiled. The hydrochloric acid can therein be vaporized and separated from the by-products by distillation. However, thousands of gallons of spent pickle liquor are produced in steel mills every day. This large volume of spent pickle liquor must then be transported to a roasting facility. Once at the roasting facility, the spent pickle liquor must be heated to the high temperatures needed for distillation. The costs of fossil fuels are at record highs. Thus, the cost of both transportation and roasting have risen dramatically and some steel companies believe that regeneration have become cost prohibitive.
Furthermore, during the pickling of metal, various inhibitor compounds are added to the pickling baths. The inhibitor compounds help protect the base metal from reacting with the acid, while allowing the scale and oxides to dissolve. Some inhibitor compounds actually increase the reaction of the acid to the scale and prevent entrainment of the acid into the atmosphere.
During the roasting of spent pickle liquor, only hydrochloric acid is recovered. The inhibitor compounds are not recycled and are lost. The inhibitors must therefore be again added to the recovered acid before the recovered acid can be used in a pickling bath.
In the prior art, typical roasting techniques are disclosed in U.S. Pat. No. 4,382,916, to Beecher, entitled Method Of Preparing Hydrochloric Acid And High Purity Ferrous Sulfate Hydrate Crystals From Hydrochloric Acid Waste, and U.S. Pat. No. 4,222,997, to Beecher, entitled Method Of Recovering Hydrochloric Acid From Spent Hydrochloric Acid Pickle Waste. In such prior art techniques, hydrochloric acid is boiled under pressure. Hydrochloric acid is recovered by condensing vapors to remove hydrochloric acid and water.
In U.S. Pat. No. 4,436,681 to Barczak, entitled process For Producing Iron Oxide Weighting Material From Iron Chloride Solutions, hydrochloric acid is recovered from spent pickle liquor by injecting the pickle liquor into a roasting chamber at very high temperatures. The ferrous chloride is converted to Fe2O3 and HCl. The roasting process uses a great deal of energy and requires high maintenance costs. The Fe2O3 produced is of low marginal quality and commercial value.
Some spent pickle liquor regeneration techniques other than roasting have been explored. It has been discovered that if sulfuric acid is mixed with a spent pickling liquid of hydrochloric acid, hydrogen chloride gas is produced. This gas can be distilled out at high temperature in a procedure very similar to roasting. However, by applying a vacuum to the reaction chamber, high roasting temperatures need not be used. Rather, by reacting spent pickle liquor with sulfuric acid in a vacuum chamber, the hydrogen chloride can be recovered at only moderately elevated temperatures. Such recovery techniques are disclosed in German patent Application No. DE 4122920 A1 to Czarnowski, entitled A Method for Regenerating Pickling Liquor; and Japanese Patent Publication No. 10-130026 to Nobuyoshi Yatomi, entitled Processing Method Of Hydrochloric Acid Discard.
Although the spent pickle liquor need not be roasted, it still must be heated in a vacuum chamber. Such facilities are highly sophisticated and are impractical to create at a steel mill. Consequently, the spent pickle liquor must be transported to specialized recovery facilities. The recaptured hydrochloric acid must then be transported back to the steel mill for use. It is the storage and transportation of this hazardous material that causes such regeneration processes to be economically unattractive.
Another disadvantage of the vacuum chamber regeneration method is that once the hydrogen chloride gas is extracted, a muck of sulfuric acid and ferrous sulfate is left over. The sulfuric acid can be separated from the ferrous sulfate in a secondary procedure, but then the used sulfuric acid is left over. The sulfuric acid can be separately transported to a steel mill for reuse, however eventually it must be disposed. The problem and cost of disposing a hazardous waste is therefore not eliminated by the regeneration process, thereby undermining the primary incentive to use a regenerating process.
Yet another disadvantage of vacuum chamber regeneration techniques is that the inhibitor compounds added to the pickle liquor are not recycled and are lost. Additional inhibitor compounds must then be added to the regenerated hydrochloric acid before it can be used in pickling. This adds significantly to the cost of recycling the pickle liquor.
A need therefore exists in the art for a regenerating process for spent pickle liquor that does not require high temperature, yet produces no waste acids that require deposal. A need also exists for a method of regenerating spent pickle liquor on-site so that the costs of transporting and storing hazardous wastes can be eliminated. A need also exists for a regeneration process that recycles the inhibitor compounds added to the pickling liquor so that expensive new inhibitor compounds do not have to be constantly added. These needs are met by the present invention as described and claimed below.